By Dr. George B. Beestman
Beestman Formulations Consulting
THE TREND that most directly affects formulators is the evolution of active pesticidal compounds from small, stable, broad-spectrum molecules to large, complex molecules with specific activity, chemical instability and high cost. These compounds are often combined into synergistic combinations with one or more other compounds. Liquid and meltable compounds that would individually be formulated into emulsion-based formulations now must be combined with crystalline solid compounds that are typically formulated into finely suspended crystals in water. These complex formulations are expected to be easy to use, have a long shelf life, and be environmentally benign. Solvents must be non-volatile. Inerts must be safe and non-toxic.
Close cooperation between surfactant supplier formulators and industrial pesticide producer formulators assures that new tools keep emerging for the formulators’ toolboxes. Polymers are developed that stabilize suspensions of pesticide crystals, increase water-solubility of compounds, improve efficacy of compounds, add systematicity to compounds, and compatibilize aqueous phases with organic phases in modern complex formulations. Other polymers retain compounds onto foliage and increase mobility of very hydrophobic molecules through soil. Stable suspensions of finely dispersed pesticides must remain fully dispersed both in the container and in the spray tank.
Ultraviolet light absorbers are added to formulations and coated directly onto pesticide particles. Emulsifiers based upon sugars and other natural products are available. Emulsifiers, polymers and adjuvants add viscoelastic properties to liquid formulations for ease of use, clean pour-out from containers, and more complete clean-out from spray equipment. Emulsifiers and polymers that enhance the efficacy of glyphosate continue to be developed.
Other polymers and emulsifiers are made available to compatibilize glyphosate with many co-herbicides now being combined with glyphosate. Adjuvants and polymers that reduce volatility and off-target movement of pesticides are emerging. Granular carriers have evolved from ground, calcined clay, irregular particles, to finely powdered particles engineered into highly adsorptive spherical non-dusty granular carriers.
■ Seed Coating
A major trend in application of pesticide compounds is via coating onto seeds. Nearly every patent and patent application that reveals new pesticide compounds will contain statements that reference their use for application to seeds.
Even new herbicide compound patents and applications reference seed coating. Formulations are being created specifically for application to seeds. Again, polymers are proving to be useful seed coating formulation adjuvants, just as they have been used to enhance formulations generally. Shelf-stable liquid and dry formulation types are created specifically for coating seeds. Thermosetting polymers, hyperbranced polymers, and structured polymers are utilized to hold compounds onto seed surfaces.
Polymers may be used to coat seeds prior to application of pesticidal compounds so that those compounds are not phytotoxic to the seeds. Polymer waxes have been used to facilitate seed flow in application. Polymers that have reactive functionality are cross-linked on the seed surfaces to form films that bind the pesticides to the seed surface. Live cell inoculants have long been applied to seeds. Natural materials essential for cell survival are now being combined with polymers and adjuvants for more robust seed treatments.
■ Liquid Formulations
Rarely are pesticides or their salts sufficiently soluble in water to form aqueous solutions concentrated enough to be commercially viable. Exceptions are glyphosate and Auxin-type herbicides. Formulation adjuvants are being developed to both enhance and increase the speed of glyphosate biological efficacy. And, as mentioned earlier, compatibilize compounds to be combined with glyphosate.
Polymers and adjuvants are available to reduce crystal growth of Auxin herbicides in glyphosate and to reduce volatilization of these herbicides to reduce off-target movement. Adjuvants to prevent degradation of compounds like sulfonylureas when combined with glyphosate are emerging.
Microencapsulated pesticides are also being successfully suspended in glyphosate aqueous formulations. Emulsifiers and polymers that tolerate the salinity of glyphosate and salt solutions are evolving.
Water-insoluble pesticides are more common as well. New solvents are being developed from natural sources to dissolve and deliver these pesticides. These solvents have low volatility yet high degree of solvency. Increasingly, solvent combinations are being employed to produce stable formulations of complex modern pesticides.
Microemulsions and gel formulations are being developed. Microemulsions are optically clear thermodynamically stable systems that are being used as adjuvants as well as the carriers of pesticides in microemulsion formulations. Shelf-life and spontaneity of these formulations into water require new emulsifiers and adjuvants. Crystalline solid pesticide powders are typically milled to small size and suspended in water. However, solvent-based suspensions of pesticides are being developed, particularly for pesticides susceptible to hydrolysis.
Suspensions of crystalline powder pesticides are combined with dissolved co-pesticide compound into more complex formulations. Polymers are being developed to keep pesticide particles suspended both in the package and when diluted in the spray tank. Final particle sizes in suspension formulations are decreasing through use of novel suspending polymers and new milling technologies.
■ Dry Formulations
Conventional granular formulations applied like granular fertilizer are still a niche formulation type preferred by some growers. Innovations in granular formulations include granules that disperse to smaller particles when contacted by water or form films after contact with water. Modified clay granule carriers that reduce the phytotoxicity of the pesticide compounds to turf have been made. Non-clay granular carriers from organic natural materials are available.
Most granular formulations are water-dispersible granules prepared for spray application. New polymers and adjuvants bind powders together as non-dusty granules that disperse rapidly in water to sprayable micron-sized primary particles. These suspended compounds stay suspended in water, cling to foliage and provide high unit activity. These granules can preserve chemically unstable compounds for longer shelf-life.
■ Controlled Release
Microencapsulation of liquid and low-melting point pesticides is now a commonly used technology. Crystalline solid pesticides are also microencapsulated when dissolved in solvents. Innovations in microencapsulation continue to develop. Polymers are used to slow crystallization so that shellwalls form before compounds crystallize, allowing pesticides to be successfully encapsulated at lower temperatures and with less solvent. Unencapsulated pesticides are combined with microencapsulated pesticides. Slow-release microcapsules are combined with fast-release microcapsules to allow optimal timing of active ingredients or separate safener compounds from active pesticidal compounds. Amino acids added to the aqueous phase prior to emulsifying the pesticides for microencapsulation resulted in emulsion droplets of the nanometer size-range and nanocapsules were produced.
Direct coating of every dust spec of a pesticide crystalline powder is not possible, because pesticide particles small enough to be biologically active cannot be fluidized in air to separate particles for polymer deposition onto their surfaces. They are small enough to start taking on liquid flow properties. When such tiny particles are suspended in water, the coating polymers form a matrix of many tiny particles forming beads too large for spray application. Processing technologies distinct from common fluid bed coating now make it possible to directly coat pesticide crystalline microparticles. Coating of individual microparticles suspended in water has also been achieved. Structured polymers can be used to suspend fine particles for milling in water to the micrometer size range of agricultural pesticides. Then, the structured polymers are cross-linked onto the pesticide crystal surfaces. Polymers are used to ensure deposition of coating polymers onto the surfaces of tiny particles suspended in water and not form beads with multiple small crystals.
Polymers extruded to films, nets, nano-fibers and nano-tubes are used for controlling the release of pesticides. Polymer nanoparticles and polymer latex particles are also used as carriers of pesticides to control the rate of release. Polymers synthesized in the presence of pesticides form polymer compositions that are molecular imprints for the specific pesticide. These molecular imprinted polymers are gaining use in the formulation of pesticides. Pickering emulsions are suspensions of oil in water that are created with tiny inorganic particles covering the surface of the oil droplets rather than emulsifiers. When reactive emulsifying particles are used, these particles can be cross-linked to form inorganic shellwalls. Inorganic shellwalls are thus created to protect compounds from ultraviolet light.
The contents of this article are a snapshot of trends and developments gleaned from the United States patents and patent applications of 2011. Each patent is available at www.beestmanconsulting.com. This article covers only a portion of the challenges facing modern formulators. New emulsifiers, adjuvants, and solvents are being created to solve the more difficult formulation challenges. Developments in nanotechnology and stimulus response coatings are emerging. As we look back at formulations from the past 10 years, we can be amazed at recent developments. I believe that when we look back again in 10 years at today’s technologies we will be amazed again at the progress.
Dr. George Beestman is an internationally recognized pesticide formulations chemist with 21 years of experience with Monsanto, 10 years with DuPont, and is currently a pesticide formulations consultant and an editorial advisor to Farm Chemicals International. He holds 10 US patents, including pioneering patents in high concentration microencapsulation and coating of fine crystalline particles. He has authored four book chapters and is the editor of three Formulation Symposia books. He can be reached through his website at www.beestmanconsulting.com.